Microwave amplitude modulator-i



Jan. 31, 1956 J. F. ZALESKI ET AL MICROWAVE AMPLITUDE MODULATOR Filed Sept. 12, 1952 000905 az/v.

IN VEN TORS. JOK/A E" amt-J0 08527 CQ/M/f, J1.

WCROWAVE AMPLITUDE MODULATGi? saint F. Zaleski, Valhalla, and Robert Crane, n, (Ihappaqua, N. Y., assignors to General Precision, Labnra tory incorporated, a corporation ofNew York vApplication September 12, 1952, Serial No. $03 ,170

9 Claims. (Cl. 332-57) Various methods have been heretofore proposed for modulating microwave energy but in general these methods have the disadvantage of producing output signals in which some frequency modulation is present in addition to the desired amplitude modulation. Additionally such arrangements, which usually employ crystal elements have been limited to low power levels.

The instant invention provides an arrangement wherein a gas discharge tube is so positioned as respects a resonant cavity that when a modulation signal is applied threto a pure amplitude modulated output is produced at high power levels if so desired.

The invention is thus useful whenever it is desired to amplitude modulate microwave energy as by voice frequencies, codesignals or indeed any desired modulation signal where an output is desired which is free of frequency modulation products. Likewise it is not necessary that the microwave carrier frequency be restricted to a single frequency but may instead be composed of a band of frequencies which of course would preclude the use. of frequency modulation.

An object of the invention is to provide an arrangement for amplitude modulating microwave energy.

A further object of the invention is to provide an arrangement for producing amplitude modulation products free of frequency modulated signals.

A still further object of the invention is to provide a microwave energy modulator capable of use at high as Well as low power levels.

A further understanding of the invention may be secured from the following detailed description and associated drawings, in which:

Figure 1 depicts one embodiment of the invention.

Figure 2 is a schematic diagram of a circuit utilizing the apparatus of the invention.

Figure 3 is a graph illustrating the operation of the circuit of Fig. 2.

Figure 4 depicts a modified form of the invention.

Figures 5 and 6 are partial sectional views illustrating details of two further modifications of the invention.

Referring now to Fig. 1, a microwavecoaxial cavity comprises an outer cylindrical conductor 11 and an inner cylindrical conductor 12. The outer conductive cylinder; it is conductively closed at one end 13, the other end being effectively open. The inner conductor 1?. is threaded through the outer conductor at 14 and is terminated in a knob 16 for tuning adjustment of its length L, which should be approximately an odd multiple of one-quarter wavelength in free space corresponding to the frequency of the applied microwave energy. An annular choke 17 of conventional form is. applied to the opening for the central conductor to reduce the impedance of the passage at wall 13 to a low value and to innited States Patent crease the impedance of the other end of the passage ad jacent bearing ring 18. If a greater tuning range of the cavity is desired than is permitted by this choke 17, it may be replaced or supplemented by conventional brush fingers bearing on the central conductor 12. Two iris openings 19 and 20 are provided to couple the cavity to a suitable carrier source of microwave energy, and to abstract energy from'the cavity, respectively.

A metal cylinder 21 is clamped to the open end of cylinder 11 by a clamp ring or screw thimble 22. One end of the cylinder 21 is hermetically closed by a nonconductive disc 23 and the other end by a disc 'A central conductive rod 25 terminates at its inner end in a disc 26, and is secured and insulated in the center of disc 24 by a hermetic glass seal 27. A glasstube 2S slightly shorter than theinner length of rod 26 is se: cured to the disc 24 to serve as an ionization shield. The cylinder 21 and the two ends 23 and 24 thus form a closed chamber which may be exhausted and filled with a suitable gas such as neon, argon, nitrogen or mercury vapor, at a suitable pressure. connected respectively, to the cylinder 21 and central rod 26 permit the application of an electrical potential between these members so that the cylinder 21 and rod 26 thus constitute the electrodes of a glow discharge tube the envelope of which consists of the cylinder 21, 'disc 23 and end disc or closure 24.

In operation, microwave energy is applied by means of a suitable wave guide to the iris openinglh, setting the cavity into resonant oscillation, and the length L of the central rod 12 is adjusted to exact resonance. Modulating current is applied between the conductors 29 and 31. The microwave electromagnetic field between the cavity center conductor 12 and the outer conductor extends through the non-metallic sealing disc 23 and some distance into the space occupied by the glow discharge plasma produced by the potential gradient established between the electrodes 21 and 26, the conductivity varying in inverse relation to the modulating current. This commingling of the field and plasma produces a modulation of the microwave field corresponding to the vari ation in current flow between the. glow discharge electrodes 21 and 26 and in such a manner as not to affect the reactive values of the cavity. The amplitude of the resonation of the cavity is therefore correspondingly varied while the frequency of resonation is unaffected, its behavior being analogous to that of a series resonant wire circuit containing series resistance, the resonant frequency of which is unaffected by changes in the value of the resistance.

Fig. 2 illustrates one maner in which the modulator of the invention may be used to amplitude modulate wideband microwave energy, which cannot be frequency modulated elfectively. The circuit also serves to demonstrate the complete absence of frequency modulation.

A microwave generator 33 generates microwave energy having a frequency of say, 10,000 me. p, s. The generator frequency is varied above and below its mean value by :5 me. p. s. at any convenient frequency, for example,

60 times per second, by a sawtooth sweep circuit 34. T he generator output contains, therefore, a band of microwave frequencies 10 me. p. s. wide. This frequency spectrum is applied through a rectangular guide 36 to an iris opening of a microwave cavity 37' such asthat described in connection with Fig. 1'. The associated gas discharge chamber 38 is operated by a sinusoidal alternating current produced by generator 39 having a frequency of say, 6000 C. P. S. A rectangular guide 41 is connected to the other iris of the cavity 37 for the purpose of secur ing the amplitude-modulated microwave energy from the cavity, the guide conducting this energy to a crystal demodulator 42. Demodulated 6000 C. P. S. voltage then Conductors 29 and 31,

appears at a conductor 43 connected to this crystal demodulator.

In order to demonstrate that the modulation is entirely free of frequency modulation it is only necessary to connect a cathode ray tube 44 for periodic horizontal deflection by the output of sweep circuit 34 and for vertical deflection by connection to conductor 43.

In the absence of modulating voltage the cathode ray tube screen exhibits the curve 46, Fig. 3. represents the characteristic resonance curve of the cavity 37, the peak abscissa representing the median applied frequency of 10,000 mc. p. s. with bandwidth limits of 9995 and 10,005 mc. p.s. Upon application of the 6000 C. P. S. modulating frequency signal to the gas discharge chamber the amplitude of the energy of the cavity resonator is rapidly varied, producing on the screen of the cathode ray tube 44 a considerable number of successive vertical oscillations of varying vertical heights which have, as a maximum vertical deflection, an envelope represented by the curve 46 and for their minimum vertical deflection an envelope such as represented by the curve 50, the exact value for minimum vertical deflection depending on the peak value of gas discharge chamber current. As an example, the limiting curves 46 and 50 represent such value of peak potential applied to the gas discharge chamber as to produce 80% modulation and it is found that the peak abscissae of yall of the family of curves occurring between these limits which, of course, represent by their ordinate values the instantaneous current flow in the gas discharge chamber, remain exactly at the center frequency of 10,000 mc. p. s., thus establishing a complete absence of frequency modulation.

Fig. 4 depicits a modified form of cavity and gas discharge combination embodying the principles of this invention and having the advantage of an arrangement of parts such that the coupling between the gas discharge plasma and the microwave field is unaffected by changes in cavity tuning. The cavity is of the open-end coaxial type having an outer conductor 47, inner conductor 48 and closed end 49. The inner conductor 48 is arranged to slide into and out of the cavity for tuning purposes, and the passage in which it slides is protected by a microwave choke 51 as in the form of Fig. 1. The inner conductor 48, however, is made hollow and constitutes the container for the gas discharge chamber. The inner surface 53 of the tube conductor 48 itself serves as one gas discharge electrode, and the other electrode is constituted by an axial rod 54 supported internally of the conductor 48 by an insulating sealing disc 56. The end 55 of the axial rod 54 is shorter than the free end 52 of the tube 48 by a greater or lesser amount as desired, the desired modulation effect being slightly greater when the rod end 55 is near the tube end 52. A glass ion shield 57 is provided surrounding the conductor 48 for nearly its entire length terminating just short of the end 55 thereof. The glow discharge occurs between. the unshielded end 55 of the lamp axial conductor and the tube surface 53, and is so positioned that its plasma is near the free end 52 of the inner conductor 48. The fringe of the microwave field of the cavity therefore, comes into contact with the glow discharge plasma at the free end 52 of tube 48. This free end 52 is closed by a non-conductive disc 58 in order to seal the glow discharge chamber and permit its evacuation, this disc being pervious to the field although impervious to the plasma. Two conductors 59 and 61 are provided for the excitation of the glow discharge chamber, the conductor 59 being grounded.

Microwave energy cannot pass through the end of the central conductor 48 into the interior thereof for an appreciable distance when the tube end 52 extends beyond the central lamp electrode end 55 and therefore behaves as a hollow round wave guide rather than as a coaxial line, since the inner diameter of the conductor 48 is This curve.

4 made less than the round wave guide cutoif dimension for microwave energy of frequency appropriate for use in the cavity, energy does not propagate within the tube 48 for any appreciable distance. When the end extends to the disc 58 the microwave energy still is not propagated to any great extent down the tube 48 because the gas plasma, having high conductivity, rapidly attenuates the microwave energy. Microwave energy also is not radiated from the free end of the outer cavity conductor 47 because this conductor extends beyond the inner cavity conductor 48 and has a diameter less than the cutoff dimension for round wave guide. Microwave energy therefore is not propagated to the left in the drawing beyond the end of the inner conductor.

The form of the invention as depicted in Fig. 4 has the distinct advantage of always insuring the same coupling between the microwave field and the gas discharge plasma regardless of the relative adjustment of insertion of the central conductor 48 in the outer conductor 47 for tuning purposes. In this arrangement the central conductor itself constitutes the envelope and one electrode of the gas discharge device so that regardless of its longitudinal position as respects the outer conductor 48, the same amount of fringe of the microwave field is always intercoupled with the glow discharge plasma.

The open end 62 of the cavity of Fig. 4 may be hermetically closed asillustrated in Fig. 5, the closure 63 being either conductive or non-conductive without effect upon operation since this member is below cutoff to wave guide propagation in the absence of an inner conductor. Enclosing the cavity permits its evacuation, using conventional methods of sealing the sliding end of the inner conductor.

Fig. 6 depicts another modified form of cavity and gas discharge combination. It combines some of the advantages of the previously described forms in retaining the physical separation and independence of the cavity central adjustable element and the gaseous modulator, while securing substantial independence between the degree of coupling and the cavity rod adjustment. This result is secured by applying the modulating gaseous tube to the side wall of the resonant cavity of Fig. 1 instead of to its end. The outer cavity element 11 containing its central conductor 12 is closed by a metal or insulating disc 63. An orifice in the side wall near the end 32 of the central conductor 12 is closed with an insulating window 64, which may be of glass or other suitable material. A metal cylinder 66 is fastened to the side wall of the cavity around the window 64 so as to make good electrical contact and a gas-tight mechanical joint. A central conductor 67 terminates near window 64 in a mushroom head 68, and is provided with'a glass ion shield 69 connected to a glass end seal 71 which is hermetically sealed to the cylinder 66. Conductors 72 and 73 are for applying the modulating voltage to the modulator.

The operation of this form of the invention is similar to the form of Fig. 1, the window 64 being permeable to microwave field energy but impervious to the glow tube plasma. Interaction of the field and the plasma therefore occurs in the space between the mushroom head 68 and the window 64, with resulting modulation of the microwave energy.

What is claimed is:

1. A microwave amplitude modulator comprising, a microwave coaxial cavity including inner and outer conductors, said inner conductor being positioned coaxially with said outer conductor and terminating in a free end coupling means for exciting said cavity at its resonant frequency, a gas discharge tube having its electrodes positioned adjacent the free end of said inner conductor whereby a portion of the resonant field space of said cavity is interlinked with the plasma of said gas discharge tube, means for variably energizing said gas discharge tube, and microwave circuit means for abstracting modulated microwave energy from said cavity. a

' tube.

axially and inwardly thereof from said closed end and terminating in a free end, means for adjusting the distance between the closed end of said outer conductor and the free end of said inner conductor, a gas discharge tube having its electrodes positioned adjacent the free end of said inner conductor, shielding means for confining the plasma of said discharge tube to the space adjacent the free end of said inner conductor, means for variably energizing said gas discharge tube, and microwave circuit means for abstracting modulated microwave energy from said cavity.

3. A microwave amplitude modulator comprising, a microwave coaxial cavity having an outer cylindrical conductor conductively closed at one end and open at the other end and having an axial conductor extending from said closed end and coextensive with a portion of the axis of said outer conductor, coupling means for exciting said cavity at its resonant frequency, a gas discharge tube, means for securing said tube to the open end of said outer conductor whereby a portion of the plasma of said tube and a portion of the electromagnetic field of said cavity are coextensive, means for variably energizing said gas discharge tube and microwave circuit means for abstracting amplitude-modulated microwave energy from said cavity. I

4. A microwave amplitude modulator comprising, a

' microwave coaxial cavity having an outer cylindrical conductor conductively closed at one end and open at the other and a hollow inner conductor extending from said closed end and coextensive with a portion of the axis of said outer conductor, coupling means for'exciting said cavity at its resonant frequency, a gas discharge tube havingat least one electrode positioned within said hollow inner conductor adjacent the free end thereof, means for variably energizing said gas discharge tube, and microwave circuit means for abstracting amplitude modulated microwave energy from said cavity.

5. A microwave amplitude modulator in accordance with claim-4 in which the outer surface of said inner hollow conductor constitutes the center conductor of said coaxial cavity and the inner surface of said inner hollow conductor constitutes one electrodeof said gas discharge- 6. A microwave amplitude modulator in accordance with claim 4 in which diameter of said outer conductor is below the cutofi wavelength of such conductor considered as a hollow cylindrical wave guide at said resonant frequency. v

7. A microwave amplitude modulator comprising, a microwave coaxial cavity having an outer cylindrical conductor closed at one end thereof and a hollow inner conductor inserted therein extending axially from said closed end for an adjustable distance less than the length of said outer conductor, coupling means for exciting said cavity at its resonant frequenc an elongatedelectrode insulatively supported in the interior of said hollow inner conductor and extending for at least the greater portion of the length thereof, means providing a sealedchamber interiorly of said hollow inner conductor, a gas filling therefor whereby said hollow inner conductor and said elongated electrode constitute the electrodes of a gas discharge tube, means for applying a modulating signal between said hollow inner conductor and said elongated electrode, and microwave circuit means for abstracting amplitude modulated microwave energy from said cavity.

8. A microwave amplitude modulator in accordance with claim 7 in which said outer conductor has a diameter which is less than the cutoff wavelength of a hollow cylindrical wave guide at said resonant frequency.

9. A microwave amplitude modulator comprising, a

microwave coaxial cavity having an outer cylindrical conductor closed at one end and a hollow inner conductor projecting coaxially therewith from said closed end for an adjustable distance less than the length of said outer conductor, coupling means for exciting said cavity at its resonant frequency, an elongated electrode insulatively supported interiorly of said hollow inner conductor extending along the longitudinal length thereof and having an end longitudinally spaced from the end of said hollow inner conductor remote from the closed end of said outer conductor, a nonconductive gas seal at said last mentioned end of said inner conductor, a second gas seal extending between said hollow inner conductor and said elongated electrode, a gas filling for said inner hollow conductor in the chamber formed between said seals whereby a gas discharge tube is formed by said hollow inner conductor having as electrodes therefor said elongated electrode and said hollow inner conductor, shielding means for. confinabstracting amplitude modulated microwave energy from said cavity.

References Cited in the file of this patent UNITED STATES PATENTS 2,106,149 Linder Jan. 18, 1938 2,556,607 Wheeler June 12, 1951 2,557,961 Goldstein et al. June 26, 1951 2,603,754 Hansen July 15, 1952 

